Build-up welding indicates a welding method of welding a metal on a base material surface. The build-up welding is performed to impart characteristics such as wear resistance and corrosion resistance to the base material surface. For example, the face part of an engine valve is repeatedly put into contact with a valve seat and is thus required to have high wear resistance. On the other hand, a material having high wear resistance is generally poor in toughness, and this makes it difficult to produce the entire valve by using such a material having high wear resistance. For this reason, it has been done to use a material having high toughness for the engine valve and build up a material having high wear resistance on the face part of the valve.
Various methods have been known as the build-up welding method, but in usages requiring automation of the process, for example, a plasma powder welding method or a laser powder welding method each using an alloy powder as the filler metal has been generally employed. Also, various materials has been used for the hardfacing alloy according to the purpose, but in the case of applying an overlay for the purpose of imparting wear resistance, Co-base alloys such as Co—Cr—W alloy (for example, STELLITE (registered trademark) #6) and Co—Cr—Mo—Si alloy (for example, TRIBALOY (registered trademark) 400) have been used as the hardfacing alloy. In relation to such a Co-base alloy for build-up welding, various proposals have been heretofore proposed.
For example, Patent Document 1 discloses a Co-base alloy powder for powder hardfacing, having a spherical shape and having an oxygen amount of 0.01 to 0.50 wt % and a nitrogen amount of 0.30 wt % or less.
The document above describes that by setting the oxygen amount to 0.01 wt % or more and the nitrogen amount to 0.30 wt % or less, a blow hole in the overlay metal can be eliminated.
Patent Document 2 discloses a powder for build-up welding of an engine valve, containing, on the weight basis, C: from 2 to 2.5%, Si: from 0.6 to 1.5%, Ni: from 20 to 25%, Cr: from 22 to 30%, W: from 10 to 15%, Al: from 0.0005 to 0.05%, B: from 0.0001 to 0.05%, and O: from 0.005 to 0.05%, with the balance being Co and unavoidable impurities.
The document above describes that when the contents of C, Cr and W are increased to certain values, the same effect as the carburizing effect is exerted even when inactive gas-shielded welding is used.
Patent Document 3 discloses a sub-combustion chamber cap for a diesel engine, which is made of a Co-base heat-resistant alloy containing, in terms of wt %, Cr: from 20.0 to 30.0%, W and/or Mo: from 3.0 to 16.0%, Si: from 0.5 to 1.5%, Mn: from 0.01 to 0.5%, and C: from 0.1 to 1.5%, with the balance being Co and unavoidable impurities, though this is not an alloy powder for build-up welding.
The document above describes that by optimizing the alloy composition, high-temperature oxidation resistance and thermal shock resistance are improved.
Patent Document 4 discloses a cobalt-based hardfacing alloy containing, in terms of weight ratio, Cr: from 10 to 40%, Mo: from more than 10% to 30%, W: from 1 to 20%, Si: from 0.5 to 5.0%, C: from 0.05 to 3.0%, 0: from 0.01 to 0.1%, Al: from 0.001 to 0.12%, Fe: 30% or less, Ni: 20% or less, and Mn: 3% or less, with the balance being Co and unavoidable impurities (provided that the Co amount is from 30 to 70 wt %). 
The document above describes that:
(1) by increasing the Fe amount, toughness is enhanced and at the same time, wear resistance and opponent aggression are improved,
(2) by adding Al and controlling the O content, the hardfacing effect is improved and at the same time, generation of a blow hole at the hardfaced area can be suppressed, and
(3) by further incorporating B, external intrusion of O can be prevented, the hardfacing effect is improved and at the same time, the bead shape is enhanced.
Furthermore, Patent Document 5 discloses a Co-base saw tip composed of a 1.5C-29Cr-8.5Mo—Co alloy, a 2.5C-33Cr-18Mo—Co alloy, or a 2.2C-32Cr-1.3W-18Mo—Co alloy.
The document above describes that when a part or the whole of W in a Co—Cr—W alloy is replaced by Mo, formation of a carbide is accelerated and high corrosion resistance in an acid environment is imparted.
Hardfacing alloys are sometimes required to have a plurality of characteristics according to the purpose. For example, in the case of applying hardfacing to the face part of an engine valve, not only wear resistance but also ductility to a certain extent are required for the hardfacing alloy. This is because when the ductility of the hardfacing alloy is low, cracking is readily generated during hardfacing and the productivity is reduced.
Out of the above-described Co-base alloys, the Co—Cr—W alloy has high ductility and good weld cracking sensitivity, because the hardening phase is a Cr-based carbide. However, the Co—Cr—W alloy has a problem that the wear resistance is relatively low and the wear volume during use is large.
On the other hand, the Co—Cr—Mo—Si alloy is excellent in the wear resistance, because the hardening phase is a Laves phase (Co3Mo2Si). However, the Co—Cr—Mo—Si alloy has a problem that the ductility is low and cracking is readily generated during hardfacing.
Also, when a certain element (for example, W) is excessively added to the Co-base alloy for build-up welding, this may cause reduction in ductility of the alloy powder or reduction in flowability of the molten alloy.
Furthermore, there has not yet been proposed any case where a hardfacing alloy is endowed with both weld cracking sensitivity equal to or higher than that of the Co—Cr—W alloy and wear resistance equal to or higher than that of the Co—Cr—Mo—Si alloy.    Patent Document 1: JP-A-62-033090 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)    Patent Document 2: JP-A-02-092495    Patent Document 3: JP-A-07-126782    Patent Document 4: JP-A-05-084592    Patent Document 5: JP-A-2001-123238